A conventional art regarding this type of communication terminal device with an echo canceller is disclosed, for example, in Patent Document 1.
As disclosed in Patent Document 1, a conventional communication system and a conventional line switching system for controlling training of an echo canceller are configured as shown in FIG. 9.
As shown in FIG. 9, the system includes a receive input terminal Rin 100 (hereinafter referred to as an “Rin 100”), a receive output terminal Rout 102 (hereinafter referred to as an “Rout 102”), a digital-analog converter 103 (hereinafter referred to as a “D/A 103”), a two-wire/four-wire conversion hybrid circuit 104, a telephone set 105, an analog/digital converter 106 (hereinafter referred to as an “A/D 106”), a send input terminal Sin 107 (hereinafter referred to as an “Sin 107”), an echo canceling adder 108, a send output terminal Sout 109 (hereinafter referred to as an “Sout 109”), an adaptive filter 110, an echo canceller 113, a switch 112, and a signal source 111.
The operation of this system will be described with reference to FIG. 9.
In an initial state, the switch 112 is turned to a terminal ‘a’.
When a call with the telephone sets 105 is first established, the switch 112 is turned to a terminal ‘b’. A reference signal to be used for convergence of the adaptive filter 110 is output from the signal source 111 (i.e., reference signal source 111) and input to the terminal ‘b’ of the switch 112. The reference signal output from the switch 112 is input to the adaptive filter 110 and the Rout 102. The reference signal output from the Rout 102 is converted by the D/A 103 into an analog signal, which is then input to the hybrid circuit 104.
A part of the reference signal input to the hybrid circuit 104 is send to the telephone set 105, and the other of the reference signal is returned again to the echo canceller 113 via the A/D 106. More specifically, the reference signal output from the hybrid circuit 104 is converted into a digital signal, which is then input to the adder 108 via the Sin 107. A pseudo echo y′ as an output from the adaptive filter 110 is input to the adder 108, and an echo signal y is cancelled out by the pseudo echo y′ in the adder 108. A signal output from the adder 108 is supplied to the adaptive filter 110 and the Sout 109.
After a call is established and a predetermined constant time elapses, that is, after the adaptive filter 110 of the echo canceller 113 is converged, the switch 112 is turned to the terminal ‘a’ in accordance with time control of a switch controller which is not shown in the figure.
After that, when a speech starts, a sound signal from the far end speaker is input to the Rin 100, passes through the switch 112, the Rout 102, the D/A 103, and the hybrid circuit 104, and then reaches the telephone set 105.
Meanwhile, the sound signal is partially reflected by the hybrid circuit 104, and returned to the echo canceller 113. The reflected echo signal y is canceled out by the pseudo echo y′ in the adder 108. Therefore, the speakers can talk to each other while no echo reaches the far end speaker, which is not shown in the figure, any longer.    Patent Document 1: Japanese Patent Application Kokai (Laid-Open) Publication No. 2004-23299